W. F. Cooper and W. H. Nuttall 223 



We are uow in a better position to understand the main factors upon 

 which the wetting of a solid surface depends. Let us imagine that a 

 drop of liquid has been phaced upon the surface of a sohd. Before we 

 can decide whether it will retain its form, or whether it will spread out 

 and cover the surface as a continuous film, i.e. wet the surface, con- 

 sideration must be given to the relative values of three distinct tensions^. 



Fig. 1. 



There is the surface tension of the liquid, which we will designate 

 Tj ; the surface tension of the solid T., ; and the interfacial tension of 

 the liquid to the solid Tj.,. The surface tension of the licj^uid T^ will 

 be exerted in the direction of the arrows at A (Fig. 1), and will tend to 

 maintain the spherical form of the drop. Similarly, the interfacial 

 tension T-^2 '^^i^ ^^^ exerted in the direction of the arrows at B, and this 

 tension will also tend to roll up and maintain the drop in its spherical 

 form. The surface tension T^, however, will have the opposite effect; 

 it will be exerted in the direction of the arrows at C, and will strive 

 to draw out the drop into a more and more lenticular form, and the 

 angle of contact (a) of the drop becomes smaller and smaller, until 

 finally the latter forms a continuous film over the surface of the solid. 

 For this to happen, it is not difficult to see that the surface tension of 

 the solid T^ must exceed the sum of the surface tension of the liquid T^ 

 and the interfacial tension Tjj) 



i.e. T^>T, + T,2. 



Since two of these tensions, viz., the surface tension of the solid T^, 

 and the interfacial tension Tjg, cannot be determined experimentally, 

 it is impossible to apply this equation to a simple practical test. Runtgen 

 (1878, pp. 324-8), however, has succeeded, by indirect means, in demon- 

 strating its validity for the system Rubber/Water. 



From the above equation, it is evident that for a liquid to possess 

 a high wetting power, it shall have a low surface tension (Tj) and also 



' If the drop of liquid is sui3ficiontly small, the effect of gravity need not be considered. 



